1. Field of the Invention
The present invention relates to a fixing apparatus that is applied in an electrophotographic image forming apparatus such as a copying machine, a facsimile machine, or a printer, and an image forming apparatus including the same.
2. Description of the Related Art
A fixing apparatus of this type is applied in an image forming apparatus that uses an electrophotographic method, an electrostatic recording method, a magneto-photographic method, or the like. Such a fixing apparatus heats and presses a recording sheet (such as plain paper, electrostatic recording paper, or photographic paper) onto which a toner image has been transferred, with the recording sheet sandwiched between a pair of fixing rotation members (belt, roller), and thereby fixes the toner image on the recording sheet.
For example, in fixing apparatuses as shown in FIGS. 8 to 11, an endless fixing belt 103 extends between a fixing roller 101 and a hot roller 102, the fixing roller 101 and a pressure roller 104 are pressed against each other via the fixing belt 103, and a nip region N is formed between the fixing belt 103 and the pressure roller 104. In such fixing apparatuses, the fixing roller 101, the hot roller 102, and the pressure roller 104 rotate, and the fixing belt 103 revolves. A recording sheet transported to the fixing apparatus passes through the nip region N, where the recording sheet is heated and pressed. The toner image on the recording sheet is thereby fixed.
In such fixing apparatuses, a non-contact temperature sensor (not shown) detects the surface temperature of the fixing belt 103, and a heater of the hot roller 102 is controlled based on this detected surface temperature, thereby adjusting the surface temperature of the fixing belt 103 in the nip region N, for instance.
For example, JP 2006-243029A (hereinafter, referred to as Patent Document 1) discloses a fixing apparatus that detects the surface temperature of the fixing belt using a non-contact temperature sensor, in a region E where the fixing belt 103 is in contact with the hot roller 102 (a region where the fixing belt faces the hot roller, the region being near where the fixing belt starts to wind around the hot roller) as shown in FIG. 8.
In the case where the surface temperature of the fixing belt 103 is detected in the region (see reference sign E in FIG. 8) where the fixing belt 103 is in contact with the hot roller 102, such as with the fixing apparatus shown in FIG. 8, the detected surface temperature of the fixing belt 103 approximately corresponds to the surface temperature of the hot roller 102. In a fixing apparatus such as shown in FIG. 8, the surface temperature of the fixing belt 103 changes due to the influence of ambient temperature at the same time as the fixing belt 103 separates from the hot roller 102 after revolving therearound. Specifically, even if the surface temperature of the fixing belt 103 that approximately corresponds to the surface temperature of the hot roller 102 is detected, the detected surface temperature greatly differs from the surface temperature of the fixing belt 103 in the nip region N. Accordingly, it is not possible to accurately adjust the surface temperature of the fixing belt 103 in the nip region N based on the detected surface temperature of the fixing belt 103.
In JP 2006-235604A (hereinafter, referred to as Patent Document 2) discloses a fixing apparatus that detects the surface temperature of the fixing belt using a non-contact temperature sensor, in a region G where the fixing belt 103 is in contact with the fixing roller 101 on the downstream side in the revolution direction of the fixing belt 103 (see the arrow in FIG. 9) relative to the nip region N, as shown in FIG. 9. Also, Patent Document 2 discloses a fixing apparatus that detects the surface temperature of the fixing belt using a non-contact temperature sensor, in a portion F (a region between the hot roller and the fixing roller) that is positioned on the upper portion of the upper and lower portions of the fixing belt that oppose to each other between the fixing roller 101 and the hot roller 102, as show in FIG. 10.
In the case where the surface temperature of the fixing belt 103 is detected on the downstream side in the revolution direction of the fixing belt 103 relative to the nip region N (see reference sign G in FIG. 9 and reference sign F in FIG. 10), such as with the fixing apparatuses shown in FIGS. 9 and 10, the detected surface temperature of the fixing belt 103 is the surface temperature after the revolving fixing belt has passed through the nip region N. In the fixing apparatuses as shown in FIGS. 9 and 10, the surface temperature of the fixing belt 103 gradually falls due to the influence of ambient temperature at the same time as the fixing belt 103 separates from the hot roller 102 after revolving therearound. Moreover, the surface temperature of the fixing belt 103 falls due to the heat of the fixing belt 103 being lost in the processing of fixing a toner image onto a recording sheet in the nip region N. Accordingly, in the case where the surface temperature of the fixing belt 103 is detected on the downstream side in the revolution direction of the fixing belt 103 relative to the nip region N, the detected surface temperature greatly differs from the surface temperature of the fixing belt 103 in the nip region N. Therefore, it is not possible to accurately adjust the surface temperature of the fixing belt 103 in the nip region N based on the detected surface temperature of the fixing belt 103.
Moreover, Patent Document 2 discloses a fixing apparatus that detects the surface temperatures of the fixing belt using non-contact temperature sensors, in the region E where the fixing belt 103 is in contact with the hot roller 102 (a region where the fixing belt faces the hot roller, the region being near where the fixing belt starts to wind around the hot roller) and the region G where the fixing belt 103 is in contact with the fixing roller 101 on the downstream side in the revolution direction of the fixing belt 103 (see the arrow in FIG. 11) relative to the nip region N, as shown in FIG. 11.
Also, in the fixing apparatus shown in FIG. 11, the temperatures detected by the non-contact sensors are the temperature on the downstream side in the revolution direction of the fixing belt 103 (see the arrow in FIG. 11) relative to the nip region N, that is, the surface temperature after the revolving fixing belt 103 has passed through the nip region N. Thus, it is not possible to accurately adjust the surface temperature of the fixing belt 103 in the nip region N due to the same reasons as those of the fixing apparatuses shown in FIGS. 9 and 10.